In many inkjet printheads, internal electrodes that are used to activate the printhead and create ink droplet ejection come into contact with the ink in the printhead in use. Problems can arise when using electrically conductive inks (aqueous and non-aqueous), and printhead failures are often observed with conductive inks, particularly in piezoelectric printheads. This places constraints on the inks that can be used in the printheads.
The present inventors have investigated this problem in an attempt to determine the cause of printhead failures.
The inventors have observed that the application of a voltage to a conductive ink can cause destabilisation of the ink and electrolysis, which in turn can generate gas bubbles and/or particulate materials which can block inkjet printhead nozzles, leading to printhead failure. This effect is particularly marked in printheads where the ink provides an electrically conductive pathway between oppositely charged electrodes in a piezoelectric printhead.
Further, the inventors have observed that where current can leak from printhead electrodes, higher voltage and more energy is required to achieve the same ejection force from the piezoelectric material. In extreme cases, high ink conductivity can lead to short circuits with consequential damage to electronic components of the printhead, leading to printhead failure.
The present inventors have realised the importance of electrically insulating the printhead electrodes from the ink, and have investigated approaches to achieving effective electrical insulation.